The short answer is... when she's okay with or wants a pregnancy or a sexually transmitted infection.

Here's the longer answer, since it's a bit more complicated than that.

To get you up-to-speed with the basic facts, when it comes to pregnancy, of every 100 women who have vaginal intercourse without using any reliable method of birth control for a year, around 80-90 of those women become pregnant according to most statistics. In other words, the vast majority of women who have intercourse without any protection do become pregnant in a relatively short period of time. Understand as well, that rates of unplanned or unwanted pregnancy are considerably higher for younger women, who tend to be more fertile than their older counterparts: rates of unplanned pregnancy are considerably higher for women ages 15-24 than for those over 24: in the United States, close to around 60% of all unplanned pregnancies happen just for women in that 9-year span between 15-24, as compared to for women in the 20-year-age span between 24 and 44. That difference is partly due to differences in fertility, but also because younger women have sex without reliable contraception, or without using contraception properly and consistently, more often than older women.

In terms of sexually transmitted infections, in the United States, the group at the highest risk of STIs, and with the fastest growing rate has been young adults for a while now: around half of all new sexually transmitted infections in the states each year are in people between the ages of 15 and 24. Around one in every four people in that age group has, has had or will get an STI. It's not rocket science to figure out why that is: young adults practice safer sex less infrequently, get less sexual healthcare, do tend to switch partners more quickly, and also tend to be the least informed or in the the most denial about their STI risks. As well,young adult women are at a greater risk of STIs just because of their anatomy, since their cervical cells haven't finished developing yet.

(It stands to mention that with both pregnancy and STIs, another reason younger women are often at greater risk or have greater rates of either is due to sexual or interpersonal violence or abuse: sexual or other abuse, rape and sexual coercion.)

What does all of this mean? If you don't want to become pregnant or get an STI, you have to protect yourself.

Obviously, the most clean-cut way to do that is to hold off on sex until you can deal with those risks or afford to take them. If you don't have sex (and I don't just mean intercourse), you're not going to be at risk for the possible products of sex. Most folks already know that.

But most people don't and won't hold off on sex for a whole lifetime. Most women also don't want to (or can't afford to, financially or otherwise) become pregnant every single time they have sex or risk pregnancy every single time they have sex. Pretty much no one wants to risk STIs every time they have sex. As well, while certain lifestyle choices like choosing to only have one sexual partner with whom you are exclusive for the whole of your life (and that other person making that same choice) can usually make your risks for STIs low, if that person is opposite sex, the risk of pregnancy will still exist with intercourse. And the vast majority of people do not or will not have only one sexual partner for the whole of their lives.

You also have the option of putting intercourse on the shelf until you feel more able to risk pregnancy or until pregnancy is wanted. Many sexual activities are just as pleasurable as intercourse, if not more so, and don't pose any risk of pregnancy. However, many of them will still present possible risks of sexually transmitted infections, so doing those unprotected can and often does still present those risks.

Your other option if you want to be safe from pregnancy and STIs -- and these things are options even if you make one of the two above choices -- is to only have intercourse when you are using a reliable method of birth control and when you are practicing safer sex. We know, from the standpoint of science, that these things, practicing safer sex and using contraception, have been shown, over time, to be the only truly reliable things people who are sexually active can do to prevent pregnancy or disease. Getting married doesn't reduce those risks as well, choosing only to be with people of a given age or who look this way or that doesn't give us any protection, nor does waiting to have sex until a given age. The only thing proven to really reduce those risks when people are sexually active are sound contraception used consistently and properly and a trio of safer sex practices.

So, if you want to have intercourse and prevent pregnancy, you're going to want to choose one or more than one reliable method of contraception. We've built this tool here at Scarleteen to help you discover which methods are likely to be best for you:Birth Control Bingo!

If, when you're asking about when it is safe, you're asking about what time in a woman's cycle she is more or less likely to become pregnant, the method you are asking about is natural family planning or fertility awareness, sometimes abbreviated as NFP or FAM. For older women with very regular cycles who can chart their fertility every day (not all women have the same cycle, so to find out when you are most and least fertile, you have to chart your own fertility over time), that can be a reliable method of contraception, but it is generally not one advised for women or couples who really do not want a pregnancy. And for younger women who have irregular cycles, who can't chart every day without fail, and/or who don't have one partner they know will comply with the periodic abstinence FAM entails, it's not usually a very good choice as a sole method. You're likely to need something more reliable, less involved and something easier for you to use and tougher to goof up.

In order to reduce your risks of sexually transmitted infections, it's also never wise to go without protection, especially in newer partnerships (where you have been together for less than six months). What we know has been shown to reduce those risks is for any two people having sex to:

Use latex barriers (condoms, dental dams, gloves) for all oral, vaginal and/or anal sex for the first six months of a sexual relationship,

Have both partners get a full screening for all STIs at the start of that time period, then at the end of it, and

For both partners to be monogamous (sexually exclusive to each other) during that time period.

If, at the end of that six month period, both people are staying and planning to stay monogamous AND both had negative screenings, we know that sex without latex barriers after that point poses few risks of STIs. For more thorough information on how to practice safer sex, including how to negotiate it, see here: Safe, Sound & Sexy: A Safer Sex How-To.

Condoms, by the way, are the only method of birth control which also help to prevent sexually transmitted infections. So, if you want to be protected from both, you're going to want to be sure to use condoms. If you want more pregnancy protection than condoms provide, you can just use another method with condoms, like a birth control pill or patch, a cervical barrier method, or, if you do feel like trying out FAM, you can use that in combination with condoms, too.

One final part of safety is more than physical: if your partner isn't a safe and sound person to have as a partner, not only are young women less likely to use protection, you're at other risks beyond just the physical ones. When we hear about sex and safety, we usually presume that to just be about pregnancy and disease. But that's not all there is to it. Our emotional safety matters too. If we choose partners who are abusive or controlling, who don't respect us or care for us, who don't care about our risks or theirs, not only are we more likely to wind up pregnant or with an STI (often because those people won't comply with their part of sex safety), we're also more likely to be harmed emotionally and interpersonally. So, who you choose to be your partner is also a really important part of when sex -- and the relationship it happens within -- is safe.

If you have a partner suggesting you go without safer sex or birth control -- or with whom you feel like you can't require those things -- and you don't want to become pregnant or get an infection, you're going to need to make clear that you have a hard limit when it comes to protecting yourself which you expect them to honor and comply with. A partner who cares about you and themselves will respect that, no problem. A partner who whines about it, or tries to get you to choose to take risks you don't want isn't someone treating you with care or consideration, and isn't someone to have as a sexual partner when it comes to your safety.

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